Browsing by Author "Partridge, Matthew"
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Item Open Access All-optical switching based on optical fibre long period gratings modified bacteriorhodopsin(Elsevier, 2017-11-24) Korposh, Sergiy; James, Stephen; Partridge, Matthew; Sichka, M; Tatam, Ralph P.All-optical switching using an optical fibre long-period gating (LPG) modified with bacteriorhodopsin (bR) is demonstrated. The switching process is based on the photo-induced RI change of bR, which in turn changes the phase matching conditions of the mode coupling by the LPG, leading to modulation of the propagating light. The effect was studied with an LPG immersed into a bR solution and with LPGs coated with the bR films, deposited onto the LPGs using the layer-by-layer electrostatic self-assembly (LbL) method. The dependence of the all-optical switching efficiency upon the concentration of the bR solution and on the grating period of the LPG was also studied. In addition, an in-fibre Mach-Zehnder interferometer (MZI) composed of a cascaded LPG pair separated by 30 mm and modified with bR was used to enhance the wavelength range of all-optical switching. The switching wavelength is determined by the grating period of the LPG. Switching efficiencies of 16% and 35% were observed when an LPG and an MZI were immersed into bR solutions, respectively. The switching time for devices coated with bR-films was within 1 s, 10 times faster than that observed for devices immersed into bR solution.Item Open Access Design and fabrication of optical fibre long period gratings for CO₂ sensing(2018-06) Barrington, James; James, Stephen W.; Partridge, Matthew; Tatam, Ralph P.This thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG COThis thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG COThis thesis investigated the repeatability of the overwrite long period grating (LPG) fabrication method and highlighted the advantage it offers in its ability to tune spectral features thus allowing the manufacture of bespoke sensors. Moreover, LPGs with periods ranging from 100 - 200 μm were written and a novel technique for mapping the transmission data was presented. This method gave a unique overview into the period mediated evolution of attenuation features, which, when designing LPGs that operate at the sensitive phase matching turning point, is invaluable. Further exploration into the overwrite method revealed that the UV irradiation duty cycle used in the fabrication of LPGs was found to influence the presence of harmonics, where a duty cycle of 25% maximised coupling to 2nd order transmission features. LPGs which possessed these additional spectral features within a small wavelength range (600 - 1000 nm) were assessed for their suitability in performing multi-parameter sensing. Ionic liquids were explored as an LPG CO₂ sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min.sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min. sensitive coating. It was shown that these materials demonstrate a refractive index change upon exposure to CO₂ which was maintained following mechanical stabilisation using a gelling agent. A coating system for applying the gelled ionic liquid to the surface of an optical fibre was developed and techniques to improve the coating deposition were explored. The sensor demonstrated an 8 nm wavelength shift in response to 20% CO₂, which was reversible by reducing the partial pressure of CO₂ for 25 min.Item Open Access Developing a proof of principle 3D-printed lab-on-a-disc assay platform.(2017-05) Tothill, Alexander M.; James, Stephen W.; Partridge, Matthew; Tatam, Ralph P.A 3D-printed microfluidic lab-on-a-disc (LOAD) device was designed and manufactured using a low cost (˜£1600) consumer grade fused deposition modelling (FDM) Ultimaker 2+ 3D printer with imbedded microfluidic channels 1 mm wide, 400 μm depth and with a volumetric capacity of approximate 23 μl. FDM printers are not typically used, or are capable, of producing the fine detailed structures required for microfluidic fabrication; in addition 3D-printed objects can suffer from poor optical transparency. However, in this work, imbedded microfluidic channels were produced and the optical transparency of the device was improved though manufacture optimisation to such a point that optical colourimetric assays can be performed in a microfluidic cuvette device with sample path length of 500 μm and volumetric capacity of 190 μl. When acetone vapour treatment was used, it was possible to improve transparency of plastic samples by up to a further 30%. The LOAD device is capable of being spun using an unmodified optical disc drive (ODD), demonstrating the centrifugation based separation of plasma from whole blood in a low-cost FDM 3D-printed microfluidic LOAD device. A cholesterol assay and glucose assay was developed and optimised using cholesterol oxidase (ChOx) or glucose oxidase (GlOx) respectively and horseradish peroxidase (HRP) for the oxidative coupling of chromotropic acid (CTA) and 4-aminoantipyrine (AAP). This produced a blue quinoneimine dye with a broad absorbance peaking at 590 nm for the quantification of cholesterol/glucose in solution. The colourimetric enzymatic cascade assays were developed for use within low-cost FDM 3D-printed microfluidic devices to demonstrate the capabilities and functionality of the devices. For comparison, the assay was run in standard 96 well plates with a commercial plate reader. The results demonstrated that the quantification of 0-10 mM glucose solution using a 3D-printed microfluidic optical device had a performance comparable to a plate reader assay; glucose assay in whole blood samples R² = 0.96.Item Open Access Development and application of optical fibre strain and pressure sensors for in-flight measurements(IOP Publishing, 2016-09-16) Lawson, Nicholas J.; Correia, Ricardo N.; James, Stephen W.; Partridge, Matthew; Staines, Stephen E.; Gautrey, James E.; Garry, Kevin; Holt, Jennifer C.; Tatam, Ralph P.Fibre optic based sensors are becoming increasingly viable as replacements for traditional flight test sensors. Here we present laboratory, wind tunnel and flight test results of fibre Bragg gratings (FBG) used to measure surface strain and an extrinsic fibre Fabry–Perot interferometric (EFFPI) sensor used to measure unsteady pressure. The calibrated full scale resolution and bandwidth of the FBG and EFFPI sensors were shown to be 0.29% at 2.5 kHz up to 600 με and 0.15% at up to 10 kHz respectively up to 400 Pa. The wind tunnel tests, completed on a 30% scale model, allowed the EFFPI sensor to be developed before incorporation with the FBG system into a Bulldog aerobatic light aircraft. The aircraft was modified and certified based on Certification Standards 23 (CS-23) and flight tested with steady and dynamic manoeuvres. Aerobatic dynamic manoeuvres were performed in flight including a spin over a g-range −1g to +4g and demonstrated both the FBG and the EFFPI instruments to have sufficient resolution to analyse the wing strain and fuselage unsteady pressure characteristics. The steady manoeuvres from the EFFPI sensor matched the wind tunnel data to within experimental error while comparisons of the flight test and wind tunnel EFFPI results with a Kulite pressure sensor showed significant discrepancies between the two sets of data, greater than experimental error. This issue is discussed further in the paper.Item Open Access Development of the Cranfield University Bulldog Flight Test Facility(Cambridge University Press, 2017) Lawson, Nicholas J.; Correia, Richardo N.; James, Stephen W.; Gautrey, James E.; Staines, Stephen E.; Partridge, Matthew; Tatam, Ralph P.Cranfield University’s National Flying Laboratory Centre (NFLC) has developed a Bulldog light aircraft into a flight test facility. The facility is being used to research advanced in-flight instrumentation including fibre optic pressure and strain sensors. During the development of the test bed, computational fluid dynamics (CFD) has been used to assist the flight test design process, including the sensor requirements. This paper describes the development of the Bulldog flight test facility, including an overview of the design and certification process, the in-flight data taken using the installed fibre optic sensor systems and lessons learned from the development programme, including potential further applications of the sensors.Item Open Access Dissolved oxygen sensing using an optical fibre long period grating coated with hemoglobin(Institute of Electrical and Electronics Engineers, 2016-08-01) Partridge, Matthew; James, Stephen W.; Tatam, Ralph P.A long period grating fiber optic sensor coated with hemoglobin is used to detect dissolved oxygen. The sensitivity of this sensor to the ratio of dissolved carbon dioxide to dissolved oxygen is demonstrated via the conversion of carboxyhemoglobin to oxyhemoglobin on the sensor surface. The sensor shows good repeatability with a %CV of less than 1% for carboxyhemoglobin and oxyhemoglobin states with no measurable drift or hysteresis.Item Open Access The effect of UV irradiation duty cycle on the 2nd harmonic coupling efficiency in optical fiber long period gratings(Elsevier, 2018-08-10) Barrington, James; Partridge, Matthew; James, Stephen; Tatam, Ralph P.Long period gratings (LPGs) as a sensing platform have the potential for multi-parameter measurement through the utilization of 2nd order coupling resonance bands. Although the current literature has produced LPGs with 2nd order resonance bands, the fabrication parameters required to generate these features have not been elucidated. Here, using UV irradiation via the point-by-point method, it is shown that by varying the duty cycle it is possible to fabricate LPGs that exhibit 2nd, 3rd, and 4th order coupling resonance bands. Fabrication of LPGs with a 25% or 75% duty cycle produces distinct 2nd order resonance bands, which are not observed when a 50% duty cycle is adopted.Item Open Access Fabrication and optimisation of a fused filament 3D-printed microfluidic platform(IOP Publishing: Hybrid Open Access, 2017-02-15) Tothill, Alexander M.; Partridge, Matthew; James, Stephen W.; Tatam, Ralph P.A 3D-printed microfluidic device was designed and manufactured using a low cost ($2000) consumer grade fusion deposition modelling (FDM) 3D printer. FDM printers are not typically used, or are capable, of producing the fine detailed structures required for microfluidic fabrication. However, in this work, the optical transparency of the device was improved through manufacture optimisation to such a point that optical colorimetric assays can be performed in a 50 µl device. A colorimetric enzymatic cascade assay was optimised using glucose oxidase and horseradish peroxidase for the oxidative coupling of aminoantipyrine and chromotropic acid to produce a blue quinoneimine dye with a broad absorbance peaking at 590 nm for the quantification of glucose in solution. For comparison the assay was run in standard 96 well plates with a commercial plate reader. The results show the accurate and reproducible quantification of 0–10 mM glucose solution using a 3D-printed microfluidic optical device with performance comparable to that of a plate reader assay.Item Open Access Fibre optic sensors with biologically active coatings for the detection of cells.(Cranfield University, 2019-05) Rendon Romero, Antonio; James, Stephen W.; Tatam, Ralph P.; Partridge, MatthewThe purpose of this thesis is to develop a rapid, sensitive and selective optical sensor for Campylobacter jejuni detection as it is the most common cause of foodborne illness in humans. The optical sensor platform was based on the optical fibre long period grating (LPG). The LPG fabrication technique chosen was the point-by-point method, involving the UV irradiation of photosensitive doped optical fibre. This technique allows the tailoring of the sensor platform to the requirements of the application. Polyclonal antibodies were selected as the material which can selectively immobilise bacterial cells on to the surface of the fibre optic. Methodologies for reliable and repeatable coating of this material (antibodies) onto the surface of the sensor platform were developed. Two methods to integrate the antibodies to the surface of the fibre, adsorption and covalent binding, were explored. Bovine serum albumin was selected as the material to block the sites on the surface of the fibre not covered by the antibodies, with the aim to prevent non-specific adsorption. The sensor was tested in a direct assay using bacterial samples at different concentrations. The sensitivity of the sensor was evaluated using different concentrations of the target bacteria in a direct assay and multiple repetitions, achieving a limit of detection of 10⁴ and 10³ CFU/mL (colony-forming unit (CFU), a measure of viable cells in a sample) for the sensor created using adsorption and covalent binding of antibodies, respectively. The selectivity of the sensor was explored by testing the sensor against different bacteria in a direct assay and multiple repetitions. The response of the sensor was 100% for Campylobacter jejuni (target bacteria), 22.77% for Listeria monocytogenes, 9.47% for Salmonella typhimurium and 3.01% for Escherichia coli. The enhancement of the sensitivity of the sensor using cell staining was explored. Cell staining induces a change in the refractive index of the cell, enhancing the signal detected, improving the limit of detection by one order of magnitude to 10² CFU/mL.Item Unknown Fibre-optic sensors with molecular coatings(Cranfield University, 2012-12) Partridge, Matthew; James, Stephen W.; Tatam, Ralph P.; Higson, Seamus P. J.The intrinsic stability of fibre optic based sensing systems offer a platform that is suited to hazardous waste detection in a wide range of environments. Over the last few years Cranfield University has been working on the development of chemical sensors using optical fibres in combination with a group of chemical recognition molecules called calixarenes. Calixarenes semi-selectively with a range of solvents of interest makes them useful for chemical detection systems. This work has primarily been focused on the use of calixarenes in sensing benzene and other hazardous solvents. However, this approach could potentially be expanded for use in a wide range of chemical and even biological recognition systems. The initial aim of this project was to build on the previous work in fibre optic sensing at Cranfield and explore approaches to improve and extend the performance of the sensor system. The project first focused on improving the techniques used in the Langmuir-Blodgett (LB) deposition of calixarenes. Initial studies in this area highlighted one critical experimental error associated with the use of dry Wilhelmy plates to monitor the surface pressure of the Langmuir film. Dry filter paper plates take up to 2 hours to give stable data, with a drift of up to 10% in the measured surface pressure. It is shown that this problem can be avoided by using pre-soaked plates. To provide an alternative to the Wilhelmy plate surface pressure senor, an optical fibre surface pressure sensor was developed, measuring changes in the meniscus forming properties of a liquid. The sensor consists of a tapered single mode silica fibre, mounted with a small curvature and positioned with the tapered region of the fibre immersed in the water. The performance of the fibre optic sensor is comparable with that of the conventional Wilhelmy plate surface pressure sensor showing linearity of greater than 0.9. Following the analysis of the experimental systems used in the construction of the sensors, the project then focused on the chemistry of the materials and their suitability for LB coating. A variety of these materials were spread as Langmuir monolayers and their behavior upon compression measured. Long chain-substituted resorcinarenes gave more stable monolayers than their short chain analogues. The incorporation of long chain surfactants led to large increases in surface area, demonstrating that both resorcinarenes and surfactants are located at the water surface, except for one system where a bilayer structure is potentially formed. Further work on the behavior of the materials involved the alteration of the dipole-dipole interaction of the monolayer materials with the subphase. The modification of this interaction through the introduction of dipole altering additives, including alcohols and hydrogen peroxide, to the aqueous subphase was investigated. The resulting isotherms of the materials showed a reduction in the surface pressure and area per molecule required in order for the monolayer to reach its point of collapse. This ability to shift the point of collapse has application in the optimisation of Langmuir-Blodgett coating of surfaces. Within this project the sensing properties of a fibre sensor were also modelled extensively in order to determine the theoretical sensing limits of a fibre optic vapour sensor. The model showed that the sensing goals of 1ppm originally envisaged for this project were unobtainable due to the low number of gas molecules interacting with the sensor. However, this led to the proposal of a new application of the system in sensing contaminants in water, where the same limitations would not apply. The results for the sensor system tested in water show how significantly more sensitive the system is to toluene contamination in water than it is to toluene vapour. These results demonstrate the utility of the developed system for many pollutant-sensing applications, include crude oil detection.Item Unknown Highly sensitive and selective biosensor based on graphene oxide coated long period grating(SPIE, 2017-02-28) Partridge, Matthew; James, Stephen; Shaikh, M. S.; Liu, Chen; Zhu, W. D.; Chena, X.We propose an optical fiber immunosensor based on graphene oxide coated dual-peak long period grating (GO-dLPG), in which GO-IgG linking layer is used for rapid immunoassays. The binding interaction between antibody and antigen produced a detectable optical signal in terms of grating resonant wavelength shift, which was proportional to the analyte concentration. By deposition of GO overlay, the bulk RI sensitivity of dLPG was enhanced around 150%. The GO-coated dLPG was biofunctionalized by the immobilization of IgG to generate the biosensor. The IgG-bound GO-dLPG was used to detect the anti-IgG and anti-PSA, respectively, demonstrating high sensitivity and selectivity. The GO-dLPG biosensor can be further developed as a biosensing platform with advantages of label-free, real-time and low limit of detection.Item Open Access IEEE sensors 2016 poster(Cranfield University, 2016-10-21 15:18) Barrington, James; James, Stephen; Tatam, Ralph; Partridge, MatthewConference poster for IEEE sensors 2016 conference. Data can be found using the referenced URL.Item Open Access Interferometric fibre optic sensors incorporating photonic crystal fibre, for the measurement of strain and load(Cranfield University, 2016-03) Manders, Mark; James, Stephen W.; Partridge, Matthew; Tatam, Ralph P.Strain sensing is important in numerous fields such as: structural health monitoring [1], manufacture of composites [2], and civil engineering [3]. For many of these fields fibre optic based sensors have been utilised due to their numerous advantages, that will be described in Chapter 2. In this thesis I will described the production of three new fibre optic based strain sensors: a microcavity based in-fibre Fabry-Perot etalon (Chapter 4), a birefringent photonic crystal fibre (PM-PCF) based Michleson-interferometer (Chapter 5), and a polarisation maintaining fibre (PMF) based Michleson-interferometer (Chapter 6). In this chapter we will describe the aim of this work, the novelty of this work, and how this work is presented in this thesis.Item Open Access Long period grating based toluene sensor for use with water contamination(Elsevier Science B.V., Amsterdam., 2014-11-01T00:00:00Z) Partridge, Matthew; Wong, Rebecca; James, Stephen W.; Davis, Frank; Higson, Seamus P. J.; Tatam, Ralph P.A demonstration of the use of a calix[4]resorcinarene coated optical fibre long period grating sensor for the detection of toluene in water is presented. Monitoring water quality both for domestic use and around industrial sites is critical to the preservation of clean water provision. Here we show that, by using a fibre optic based sensor system, water quality monitoring can be carried out without the requirement for water sampling or pre-concentration. The results presented demonstrate that this proof-of-concept sensor is capable of sensing ∼100ppm concentrations of toluene with semi-selectivity and low (<10ppm) variation.Item Open Access Modifying monolayer behaviour by incorporating subphase additives and improving Langmuir–Blodgett thin film deposition on optical fibres(Elsevier, 2014-01-17) Partridge, Matthew; Wong, Rebecca; Collins, Mike; James, Stephen W.; Davis, Frank; Tatam, Ralph P.; Higson, Seamus P. J.Experiments showing the possibility of modifying the behaviour of calix[4]resorcinarene monolayers at the air–water interface and optimising the deposition of multilayer coatings onto optical fibres are presented. The nature of the subphase is fundamental to the behaviour of monolayers and their utility in coating and sensing applications. Here we show initial studies exploring the modification of the calix[4]resorcinarene monolayer–water interaction through the introduction of dipole altering alcohol additives to the aqueous subphase. We explored the effect of this modification for three small alcohols. The resulting isotherms of the materials showed a reduction in the surface pressure and area per molecule required in order for the monolayer to reach its point of collapse. Incorporation of alcohols shifted the point of collapse, leading to the application of ethanol being successful in improving the transfer of material via Langmuir–Blodgett coating onto optical fibres at lower pressures. This method may prove useful in allowing greater control over future sensor surface coatings.Item Open Access Multi-parameter measurements using optical fibre long period gratings for indoor air quality monitoring(Cranfield University, 2017-01-05 16:45) Hromadka, Jiri; Davis, Frank; Korposh, Serhiy; Partridge, Matthew; James, Stephen; Tatam, RalphData underlying the results presented and discussed in: Multi-parameter measurements using optical fibre long period gratings for indoor air quality monitoring Jiri Hromadk, Sergiy Korposh, Matthew C. Partridge, Stephen W. James, Frank Davis, Derrick Crump, Ralph P. Tatam Published in Sensors and Actuators B 2017. (http://dx.doi.org/10.1016/j.snb.2016.12.050) Note that the description of the instrumentation and conditions pertinent to each file are detailed within the paper. The filenames correspond to the corresponding figure in the paper figure.Item Open Access Multi-parameter measurements using optical fibre long period gratings for indoor air quality monitoring(Elsevier, 2016-12-11) Hromadka, J.; Korposh, Sergiy; Partridge, Matthew; James, Stephen W.; Crump, Derrick; Davis, Frank; Tatam, Ralph P.An array of three long period gratings (LPGs) fabricated in a single optical fibre and multiplexed in the wavelength domain was used to measure simultaneously temperature, relative humidity (RH) and volatile organic compounds (VOCs), which are key indoor air quality (IAQ) indicators. Each LPG sensor was designed with optimised response to a particular measurand. The first, with no surface modification, was used to measure temperature. The second, modified by a mesoporous coating of silica nanoparticles (SiO2 NPs), was used to measure RH and the third, modified with a coating of SiO2 NPs infused with a functional material, p-sulphanatocalix[8]arene (CA[8]), was employed to monitor VOC concentration. The LPGs were fabricated with periods such that they operated at or near the phase matching turning point. The sensors were calibrated in the laboratory and the simultaneous measurement of the key indoor air quality parameters was undertaken in laboratory and office environments. It was demonstrated successfully that the data produced by the LPG sensor array under real conditions was in a good agreement with that produced by commercially available sensors. The average differences between values obtained by the optical fibre sensor and standard temperature and RH sensors were better than 0.5 °C and 5% respectively. Further, the potential application of fibre optic sensors for VOC detection at high concentrations has been demonstrated.Item Open Access Overwrite fabrication and tuning of long period gratings(Optical Society of America: Open Access Journals / Optical Society of America (OSA), 2016-09-19) Partridge, Matthew; James, Stephen W.; Barrington, James; Tatam, Ralph P.The central wavelengths of the resonance bands are critical aspect of the performance of long period gratings (LPGs) as sensors, particularly for devices designed to operate near the phase matching turning point (PMTP), where the sensitivity to measurements can vary rapidly. Generally, LPGs are characterized by their period, but the amplitude of the amplitude of the index modulation is also an important factor in determining the wavelengths of the resonance bands. Variations in fabrication between LPG sensors can increase or decrease the sensitivity of the LPG to strain, temperature or surrounding refractive index. Here, the technique of overwritten UV laser fabrication is demonstrated. It is shown that, on repeated overwriting, the resonance bands of an LPG exhibit significant wavelength shift, which can be monitored and which can be used to tune the resonance bands to the desired wavelengths. This technique is applied to periods in the range 100 to 200 µm, showing the cycle-to-cycle evolution of the resonance bands near the PMTPs of a number of cladding modes. The use of online monitoring is shown to reduce the resonance band sensor-to-sensor central wavelength variation from 10 nm to 3 nm.Item Open Access Sensitivity enhancement in low cutoff wavelength long-period fiber gratings by cladding diameter reduction(MDPI, 2017-09-13) Del Villar, Ignacio; Partridge, Matthew; Rodriguez, Wenceslao Eduardo; Fuentes, Omar; Socorro, Abian Bentor; Diaz, Silvia; Corres, Jesus Maria; James, Stephen Wayne; Tatam, Ralph P.The diameter of long-period fiber gratings (LPFGs) fabricated in optical fibers with a low cutoff wavelength was be reduced by hydrofluoric acid etching, enhancing the sensitivity to refractive index by more than a factor of 3, to 2611 nm/refractive index unit in the range from 1.333 to 1.4278. The grating period selected for the LPFGs allowed access to the dispersion turning point at wavelengths close to the visible range of the optical spectrum, where optical equipment is less expensive. As an example of an application, a pH sensor based on the deposition of a polymeric coating was analyzed in two situations: with an LPFG without diameter reduction and with an LPFG with diameter reduction. Again, a sensitivity increase of a factor of near 3 was obtained, demonstrating the ability of this method to enhance the sensitivity of thin-film-coated LPFG chemical sensors.Item Open Access Soil moisture content measurement using optical fiber long period gratings(SPIE - International Society for Optics and Photonics, 2017-04-23) Hallett, Stephen H.; Partridge, Matthew; James, Stephen; Tiwari, Ashutosh; Farewell, Timothy S.; Tatam, Ralph P.The use of an optical fibre long period grating (LPG) as a soil moisture sensor is reported. Characterization of the device in both clay and sandy soils revealed a sensitivity to moisture levels in the range 10-50%, and the results were compared with the output from a Theta probe, the standard soil moisture sensor, which measures the impedance of the soil. © (2017) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.